This application claims priority to French Patent Application No. 97 09356 filed on Jul. 23, 1997, which is herein incorporated by reference.
The present invention relates to the field of manufacture of optical components and, in particular, to the fabrication of integrated (or planar) optical components. More especially, the invention relates to the fabrication of optical components in which one or more waveguides are formed in association with one or more other optical elements aligned with the end(s) of the waveguide(s).
In the field of integrated optics it is often necessary to make a component in which one or more optical elements is aligned with the end(s) of one or more waveguides. This is the case, for example, when manufacturing a wavelength division multiplexer. Typically, in a first stage of the fabrication process, the successive layers making up the waveguides are deposited on a substrate and patterns are traced therein. Afterwards, when the waveguide structure is complete (core layer and overclad layer, perhaps with a buffer layer between the substrate and the core layer), the associated optical element is formed, aligned as desired with the ends of the waveguides, by patterning and etching the completed waveguide structure. This second stage in the fabrication process involves performing a photolithography and etching process on a thick layered structure, for example 20-30 xcexcm thick, and leads to numerous problems.
The problems inherent in the conventional fabrication process will be explained in greater detail by describing a process for fabricating a diffraction-grating-based, narrow-band, wavelength division multiplexer or demultiplexer (NBWDM), with reference to FIG. 1. Typically a diffraction-grating-based, narrow-band, wavelength division multiplexer or demultiplexer (NBWDM) comprises a diffraction grating aligned with the ends of input and output waveguides serving several channels, typically 32 or more.
According to the conventional process, a wafer typically of silicon or silica having an optically smooth surface, and typically 1 mm thick, is used as the substrate for fabrication of optical components. As illustrated in FIG. 1A, a layer 20 of silica is deposited on the substrate 10, for example by flame hydrolysis deposition, or chemical deposition processes or plasma deposition processes, etc. Germanium, titanium or the like is used to dope the silica in order to raise the refractive index thereof. (In the case where a silicon substrate is used, a buffer layer is provided on the substrate 10 before deposition of the doped silica core layer 20, for the purposes of optical isolation. The buffer layer can be obtained by thermal oxidation.)
The silica layer 20 is typically between 5 and 10 xcexcm thick, for example 6.5 xcexcm thick. The cores 25 of the waveguides, as well as a planar waveguide 28, are formed from the layer 20 by patterning in a lithography step and subsequent etching to an appropriate depth, for example 7 xcexcm in the case of using a core layer 6.5 xcexcm thick (see FIG. 1B). Alignment marks for a later lithography step are formed during this first lithography step.
Next, an overclad layer 30 of undoped silica or of silica doped with, for example, boron or phosphorus, typically 10-20 xcexcm thick is deposited by a suitable process, such as low pressure chemical vapour deposition (LPCVD), plasma enhanced chemical vapour deposition (PECVD), atmospheric pressure chemical vapour deposition (APCVD), flame hydrolysis, etc. (see FIG. 1C). Afterwards. the diffraction grating 35 is formed by patterning, during a lithography step where it is attempted to align the mask with the earlier-formed alignment marks. and etching the complete layer structure to a depth of between 20-30 xcexcm (see FIG. 1D), typically by reactive ion etching. The grating-based, narrow-band, wavelength division multiplexer or demultiplexer (NBWDM) is completed by metallisation of the grating 35 by depositing a layer 36 of aluminium or gold (see FIG. 1E).
In the conventional process, numerous problems afflict the second lithography and etching steps. The substrate warps to a significant degree. typically around 100 xcexcm for a 4xe2x80x3 (100 mm) wafer, due to the thickness of the previously-deposited core and overclad layers (17-30 xcexcm). This severe degree of warping makes it difficult to obtain a high resolution lithography, which is a serious drawback for the manufacture of narrow-band WDM components or the like where very precise definition of the etched components is required. In addition, good alignment of the masks used in the two lithography steps is required but this is rendered difficult by the loss of visibility which results from the great thickness of the overclad.
Moreover, in the conventional process, deep silica etching is required and this in itself is extremely difficult. Etching times are long, typically 5 to 10 hours, and there are severe constraints on the etching mask (high thickness, high etching resistance, low stress and high etching resolution). Furthermore, there is a loss of resolution during the etching resulting in rounding of the grating between 3 and 4 xcexcm.
In view of the above problems, the present invention provides a new process for the fabrication of optical components, dispensing with the need to perform photolithography and etching steps on such a thick layered structure.
More particularly, the present invention provides a method of forming an optical component comprising at least one waveguide and at least one optical element disposed facing the end(s) of the waveguide or waveguides, comprising the step of depositing on a substrate a core layer for forming the core of the or each waveguide, characterised by further comprising the steps of: depositing a partial overclad layer on said core layer; patterning said core layer and partial overclad layer so as simultaneously to define the optical element(s) and the core(s) of the waveguide or waveguides; and depositing a further overclad layer.
By depositing the overclad layer in two stages and defining the optical element simultaneously with the waveguides by etching through the core layer and the partial overclad layer, the method of the present invention avoids the deep etching process present in the conventional fabrication method and enables high resolution definition of the optical element to be achieved.
Furthermore, by eliminating the second lithography step included in the conventional fabrication method, the present invention produces an important simplification of the process and ensures perfect alignment of the waveguide(s) with the optical element(s).
Also, by reducing the etching depth, the requirements on the etching mask are reduced, etching time is reduced and there is a further improvement in resolution (rounding of around 1 xcexcm).
Preferably, the partial overclad layer should have a thickness in the range of 1-5 xcexcm, the precise value being determined as a function of the difference which exists between the refractive indices of the core and overclad layers. For example, a partial overclad layer of 2-3 xcexcm thickness is suitable in the case where waveguides having a relative difference between the refractive indices of the core layer (e.g. n=1.46) and the overclad layer (e.g. n=1.45) of 0.69% are used.
If the thickness of the partial overclad layer is too low, then in the resulting optical component, the optical element(s) facing the end of the waveguide(s) will oppose the waveguide cores and a small portion of overclad just above (which corresponds to the partial overclad layer) and will only handle light transmitted in these elements. However, in practice, light does not remain confined in the waveguide cores but will also propagate in the overclad layer during use, according to a Gaussian distribution. The extent of this propagation outside the core layer increases as the difference between the refractive indices of the core and overclad layers become closer or decreases.
Thus, to minimise optical losses, it is desirable that the optical component(s) should oppose not only the waveguide cores but also a portion of the overclad sufficiently thick to ensure that the majority of the light propagating outside the core layer is located in this portion of the overclad layer. Because the optical elements are formed by etching the partial overclad layer, and face the portion of the overclad layer which corresponds to the partial overclad layer, it is thus necessary that the partial overclad layer should have a thickness such that the majority of the light propagating outside the core layer is locate in the partial overclad layer.
On the other hand, if the partial overclad layer is made too thick, then the advantages of the present invention (better resolution, less warping) begin to be lost.
By performing the etching step through the core layer and a partial overclad layer 1-5 xcexcm thick, good efficiency of the resulting optical component is maintained.
The present invention further provides an optical component comprising: at least one waveguide, and a grating formed by etching and disposed facing the end(s) of the waveguide or waveguides, wherein the core(s) of the waveguide(s) is or are made of silica, preferably doped silica, and the rounding of the grating due to etching is  less than 3 xcexcm and, preferably, is approximately equal to or less than 1 xcexcm.
Further features and advantages of the present invention will become apparent from the following description of the preferred embodiments thereof, given by way of example, and illustrated in the accompanying drawings, in which: